Two ways of writing powerset which produce the same ordering. Have a go at proving it.

powerset.py

def powerset(list):
    if not list:
        yield set()
        return
    *rest, last = list
    yield from powerset(rest)
    for subset in powerset(rest):
        yield subset | {last}

def powerset2(list):
    if not list:
        yield set()
        return
    first, *rest = list
    for subset in powerset2(rest):
        yield subset
        yield subset | {first}



numbers = [2, 5, 9]
print (list(powerset(numbers)))
print (list(powerset2(numbers)))